Add an option to allow invalid certificates when using TLS

[dampee]

While everyone would (and should) opose against this, it's pretty convinient in dev and test environments to allow the mail servers to have invalid or expired certificates. Or sometimes you just a malconfigured mailserver which you need to use.

I know this is a bad idea. But I still think there might be some almost valid use cases.

[dampee]

This is a duplicate of #89